数据库驱动程序类动态加载 [英] Database driver-class dynamic loading

问题描述

我想用Java开发与数据库无关的应用程序。我选择了休眠作为ORM。 jdbc的问题在于，它只是一个接口，我们需要在类路径中提供db的驱动程序类。由于数据库应该是可配置的，因此我必须动态加载数据库的驱动程序类。 （用户应将驱动程序类保存在一个文件夹中，并且应动态加载它）
下面是我的代码。

I want to develop a DB-Agnostic application in java. I have chosen hibernate as ORM. The problem with jdbc is that , it is just an interface and we need the driver class of the db in the class path. Since the database should be configurable i have to go for loading the driver class of DB dynamically. (User should keep the driver class in a folder and it should be loaded dynamically ) Below is my code.

File driverJar = new File("E:\\Jomon\\backup_2017_05_25\\2.2\\WS\\2.2_1\\lib\\Drivers\\postgresql-42.1.1.jar");
URL[] urls = new URL[] { driverJar.toURL() };
URLClassLoader classLoader = new URLClassLoader(urls,DBUtils.class.getClassLoader());
Class.forName("org.postgresql.Driver", true, classLoader);

到目前为止没有错误。
但是在此之后，在初始化休眠连接时，出现错误java.lang.ClassNotFoundException：org.postgresql.Driver。

No error till now. But after this, while initializing hibernate connection, I am getting error java.lang.ClassNotFoundException: org.postgresql.Driver.

我可以知道什么是

推荐答案

最后，我自己得到了解决方案，
这里的问题是，我创建了一个新的

Finally I got the solution by myself, The problem here is , I have created a new classloader and loaded the jar into it.

休眠在系统类加载器中搜索驱动程序类，而不是在用户定义的类加载器中搜索。

The hibernate is searching for driver class in system class loader , not in the user defined class loaders.

可以通过将jar加载到系统类加载器中来解决这里的问题，如下所示。

The problem here can be solved by load the jar into system class loader as below.

File driverJar = new File("E:\\Jomon\\backup_2017_05_25\\2.2\\WS\\2.2_1\\lib\\Drivers\\postgresql-42.1.1.jar");
URL myJarFile = new URL("jar", "", "file:" + driverJar.getAbsolutePath() + "!/");
URLClassLoader sysLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();

Class sysClass = URLClassLoader.class;
Method sysMethod = sysClass.getDeclaredMethod("addURL", new Class[] { URL.class });
sysMethod.setAccessible(true);
sysMethod.invoke(sysLoader, new Object[] { myJarFile });

Class.forName("org.postgresql.Driver", true, classLoader); // Now no error in this line.

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